An essential element in digital communications is synchronization between a transmitter and a receiver. Time synchronization has to be acquired and tracked between the transmitter and the receiver. In order to acquire time synchronization, a sync word having desirable auto-correlation properties is typically used. A Barker sequence or a maximum length linear shift register sequence are examples of this type of a sync word. The sampling time, relative to the transmitted symbol, may or may not need to be updated depending on various factors. These factors include the transmitter and receiver clock accuracy, the length of the data packets transmitted and the sampling time resolution. The procedure of updating the sampling time is referred to as tracking.
The transmitter and receiver accuracy may be designated as A parts per million (PPM), the packet duration may be designated as Tpacket, and the symbol duration may be designated as TS. Based on system specification defined by the communication system operator or on specific applications, a determination may be made as to whether tracking is needed. For example, if a time-drift in the sampling time during the entire packet corresponding to (0.1*TS) is acceptable, tracking is not needed if (2* ATPacket)≦0.1*TS. Since TPacket=NTS where N is the number of symbols, the maximum number of symbols Nmax is:
 Nmax=(0.1)/(2*A)  (1)
If both the transmitter and receiver clocks have an accuracy of 20 ppm, the maximum number of symbols, Nmax, in a packet becomes 0.1/(2*20*10−6)=2500.
The sampling time resolution is another factor which determines whether the sampling time needs to be updated. Typically, the incoming signal is sampled by the receiver at a rate that is higher than the symbol rate. That is, the received signal is over sampled, at least during the reception of the sync word. The over sampling ratio (OSR) may be designated as NOSR where NOSR is the number of samples per symbol. By correlating the received sequence with the sync word, the receiver can estimate which one of the NOSR sample phases is the best one to use as the sampling time throughout the remaining part of the packet. Assuming that the best possible sample is chosen and that the samples are equidistant, then the chosen sampling time cannot vary more than TS/(2*NOSR) from the optimum sampling time. Suppose the sampling instant that is used must be less than or equal to a variable Δt (where Δt varies according to system requirements) from the optimum sampling time, this cannot be guaranteed unlessTs/(2*NOSR)+2ATsN≦Δt,  (2)where N is the packet length (expressed as a number of symbols). Referring to (2), the packet length may be increased by increasing the OSR. However, even if the OSR is made arbitrarily large, the maximum packet length cannot exceedN=(Δt/TS)/(2*A)  (3)
It is highly desirable to increase the packet length in order to decrease overhead during transmission and increase user data rate.